1. Field of the Invention
This invention relates to a method of detecting alterations in a target document with respect to an original document, and in particular, it relates to such an alteration detection method that uses shape features of characters to detect alteration.
2. Description of Related Art
A closed-loop process refers to printing an original digital document (which may include text, graphics, images, etc.), using the printed hardcopy of the document such as distributing it, copying it, etc., and then scanning a hardcopy of the document back into digital form. Authenticating a scanned digital document refers to determining whether the scanned document is an authentic copy of the original digital document, i.e., whether the document has been altered while it was in the hardcopy form. Various types of document authentication and alteration detection methods have been proposed. A goal in many document authentication methods is to detect what the alterations (additions, deletions) are. One type of document authentication method performs an image comparison of the scanned document with the original digital document. This is sometimes referred to as a pixel-based alteration detection method. In another type of document authentication method, data representing or relating to the document content are encoded in barcodes, and the barcodes are printed on the document itself to assist in document authentication later.
In conventional pixel-based alteration detection methods, the original digital image and the scanned image (target image) are compared pixel-by-pixel to determine whether there are alterations in the scanned image. The high spatial resolution of both the printer and the scanner result in high resolution scanned images. Exhaustive image comparison processing of the whole image in a conventional alteration detection method is computationally intensive.
Further, pixel-based image comparison sometimes generates false positive detections. For example, for two characters located at the same locations of the original image and the target image, even when the two characters are the same (i.e. the character has not been altered), pixel level differences can exist between the target image and the original image. Such pixel level differences, which are introduced due to the printing and scanning processes that produced the target document, can result in false positive detections of alterations.